This invention relates to a method useful in the manufacture of field emission cathode displays ("FEDs") and other cathodoluminescent color displays of a type utilizing a color screen comprising interregistered patterns of red-light-emitting, blue-light-emitting, and green-light-emitting phosphor elements (hereinafter sometimes termed "red", "blue", and "green" phosphor elements).
FIG. 1 schematically depicts an FED display 10 of a type which may embody a screen made according to the teachings of the present invention. The display 10 comprises a glass front panel 12 and a glass rear panel 14 which are joined by a glass frit cement 16. A field emitter array 18 supports a large number of field emitters which produce electron beams accelerated through control grid 20 to excite a color phosphor screen 22.
In the fabrication of such screens, the patterns of red, blue and green phosphor elements in each pattern, and each pattern relative to the other two patterns, must be laid down with extreme precision.
U.S. Pat. No. 4,891,110 describes and claims a process for depositing by electrodeposition a sequence of interregistered red, blue and green phosphor patterns. As used herein, "electrodeposition" or "electrodepositing" refers to cataphoretic or other processes utilizing a bath from which a material is deposited on an electrically charged substrate under the influence of fields created between the substrate and another electrode.
Reference may be had to FIGS. 2A-7 wherein the method of the '110 patent is illustrated schematically. In FIGS. 2A-7, substrate "S" supports a conductor "C".
The process of the '110 patent comprises forming an electrically insulative black matrix BM--sometimes termed a "black grille" or "black surround"--having formed therein first, second and third patterns of openings (OP1, OP2, OP3) corresponding to the patterns of the red, blue and green phosphor elements (FIG. 2A). The first and third patterns of openings, are then, in effect, plugged with an insulative material IN1 (FIG. 2B). A second pattern of phosphor elements P2 is cataphoretically deposited in the second pattern of openings onto the underlying substrate (FIG. 3). The first pattern of openings is then unplugged (FIG. 4) and the first pattern of phosphor elements P1 is cataphoretically deposited in the first pattern of openings (FIG. 5). The third pattern of openings is then unplugged (FIG. 6) and the third pattern of phosphor elements P3 is electrodeposited in the third pattern of openings (FIG. 7).
The three patterns of openings in the electrically insulative black matrix are formed photographically using a set of photomasters through which exposures are made. The aforedescribed plugging of the first and third patterns of openings is accomplished using photomasters which are interregistered with the photomaster used to form the composite pattern of openings in the black matrix. This assures that the plugs are accurately placed in the patterns of openings. Kinematic fixturing techniques, or other techniques well known in the art, are used to assure interregistration of the various photomasters which are used in the described processes.
While the screening process described and claimed in the '110 patent is viable, it requires an exposure step between certain phosphor electrodeposition steps.
Specifically, after the aforediscussed second pattern of phosphor elements is cataphoretically deposited in the second pattern of openings, the first pattern of openings is selectively unplugged. This is accomplished by stripping the plugs from the first and third patterns of openings, and then replugging the third pattern of openings.
The interruption of the electrodeposition operation by a photoexposure step creates a number of difficulties.
As the fixture employed to carry the substrate during the first photolithographic plugging operation cannot travel with the substrate through the electrodeposition bath, it must be detached and another fixture reattached for the exposure step comprising part of the second plugging operation.
However, a need to disassemble and reassemble the fixture and substrate, as a practical matter, eliminates the possibility of obtaining the very high tolerances associated with high resolution FED screens using cost-effective mechanical fixturing techniques. The plugging operation involves coating the screen with a photosensitive material and exposing the material to light actinic to the material in areas which corresponds to the third pattern of openings. After development, the substrate has the first pattern of openings open and the third pattern of openings plugged, permitting electrodeposition of a pattern of phosphor elements into the first pattern of openings.
Finally, the plugs are stripped from the third pattern of openings and the third pattern of phosphor elements is electrodeposited into the third pattern of openings, completing the phosphor deposition process.
Thus, it will be seen that the electrodeposition operations of the '110 patent are interrupted by an exposure step.
To reiterate, it is imperative that each phosphor pattern be interregistered precisely with the remaining two phosphor patterns. If mechanical registration techniques are employed so as to achieve manufacturing economies, the registration fixture mated with a particular screen-supporting substrate must be mechanically coupled to the substrate throughout all photoexposure steps. As noted, this need for uninterrupted mating of fixture and substrate through all exposure steps cannot be satisfied using the method of the '110 patent. This means that more costly optical registration techniques must be employed.
The process of the '110 patent thus renders impracticable process and physical segregation of the exposure operations from the electrodeposition operations. These burdens imposed on the achievement of the necessary interpattern registration, and the inability to segregate the exposure and electrodeposition operations, translates into added cost of manufacture.